My first question for you is how experienced with 3d printing are you?
If your not careful or know your settings, you could easily print your parts, such as legs and frames, way to weak, or not size things right, leading to mounting holes for motors being too big or small.
There is also the question of how big your printer is. Some models are tested and created to work at only a specific size and can’t handle resizing very well, but if you are very sure of your printing skills, this is a non issue.
As for kits, this is a solid DIY kit. It dosen’t have any 3d parts, but it does have most of everything needed to build a drone. So you could in theory print your own frame and use the parts from this kit to complete your build.
The only other things you’ll need for that Kit is a battery and charger, controller setup, and any FPV gear, which, the UnmannedTech team have recommendations in the lower part of the page.
There is plenty of tutorials online for how to connect your parts, as well as tutorials specific to that frame which include motors, ESCs and FC hookup.
Before I’d try to do a 3d printed one, I’d first recommend getting familiar either your 3d printer or building a drone. I’d hate for you and your son to get discouraged cause what seemed simple at first turned into a bit of a hassle, especially if you are unfamiliar with both 3d printing and drone building.
If you have any questions, feel free to ask, i’ll try to answer the best i can, on both 3d printing and drones.